A plate used in a xerographic process comprises a photoconductive layer mounted on a conductive base. The photoconductive layer is sensitized by uniformly distributing electrostatic charges, either positive or negative, over the surface of the layer. The photoconductive layer is a good insulator in the dark so that the electrostatic charges are retained on the surface. Areas of the photoconductive layer can then be illuminated, for instance by projecting an image on the layer, or by writing on the layer with a laser or light-emitting diode (LED). This causes the electrical resistance of the layer in the illuminated areas to be reduced. The charges in the illuminated areas are then dissipated to the conductive base. The non-illuminated areas remain electrostatically charged. The photoconductive layer is then developed, in a developing zone, by adhering oppositely-charged pigmented and/or dyed toner particles to the non-illuminated charged areas. This is carried out in a manner known in the art, disclosed, by way of example, in U.S. Pat. No. 5,003,352, assigned to the assignee of the present application.
The development of the photoconductive layer, in a developing zone, can be accomplished using a liquid toner. Liquid toner dispersions for electrophotography are generally prepared by dispersing pigmented or dyed toner particles, and natural or synthetic resins, in a highly insulating, low dielectric constant, carrier liquid. Charge control agents are added to aid in charging the pigmented and dyed toner particles to obtain the requisite charge distribution, and liquid toner conductivity, for proper image formation on the photoconductive layer. There are a variety of mechanisms for the production of charge on toner particles. In some, the charge control agents function as surfactants and alter the surfaces of the particles providing a preferential absorption of ions of opposite charge from those on the photoconductive layer.
One deficiency of some charge control agents is that after liquid toners comprising the charge control agents have been exposed repeatedly to the electric field of the development zone, the conductivity of the liquid toner becomes transiently depressed. The ability of the charge control agent to maintain a stable charge distribution becomes diminished. This is especially true in high speed electrophotographic processes wherein images must be transferred to sheets of paper running at high speed, such as 100-1,000 feet/minute or higher. In such processes, the exposure of the toner to the electric field of the development zone over a selected period of time, is far greater, than in, for instance, an office copier or other slow-speed machine.
Failure of the liquid toner charge control agents to maintain toner charge distribution within closely controlled limits results in poor print quality. Additionally, departure of the toner charge distribution from said limits can result in a loss of control of the concentration of components of the toner composition.